anodes and corrosion

[Freebee]

I lifted my boat today for the first time in 12 months. it has twin VP 290 drives, one set of duo props pitted and corroded the other untouched except slight scaring round the prop edges, both prop anodes wasted pretty much away. what should I be doing to prevent reoccurence apart from replacing the anodes, should the drives be electically bonded to each other to get equipotential? could I fit extra anodes if so where and how. Can you have too many anodes?

And final question the pitting has eaten into the prop blades,like worm holes, can this just be filled with car filler and painted over or repaired by welding?

 

[Skyva_2]

Do you leave shorepower on and is there a galvanic isolator fitted?

You can hang anodes bonded to the engine, over the side for extra protection.

There was a recent thread on this, may be worth a search.

 

[Freebee]

no shore power.

 

[jimmy_the_builder]

I had very similar problems to this with my twin outdrive installation when I bought the boat last year - one drive casing noticeably more corroded than the other.

The drives are supposed to be internally bonded together, but I found on mine that that bonding had broken down somewhere between the casing and the transom shield, so I added external bonding and this seems to have sorted the problem out.

If you don't have shorepower then you won't need a galvanic isolator, but if you have stainless props (I do) then additional anodes is definitely a good idea (I have an additional central anode on the transom which is connected to both drives).

Cheers
Jimmy

 

[davepunch]

Installing more anodes will solve the problem but you still need to identify the cause. Anodes are only really to be used for control of electroysis caused by different metals in water, it sounds to me like you have a galvanic corrosion problem caused by a voltage leak above that of zinc.

The scaring around the edges of the 'good' prop may be down to cavitation.
So assuming that there is no galvanic action on the good prop:
How is the boat mored?. Is the bad prop nearer to another vessel or a metal pile in the water, if the bad prop is connected to the good prop (wich it most definately should be) then this is good for prevention of electrolysis but bad if there are gradually reducing voltages passing athwartships (the copper wire connecting the two outdrives is a better conductor than the water creating an imbalace between props.
Not only will the vessel or metal pile next to you 'steal' the metal from your bad outdrive it will cause the anode of your good outdrive to become a cathode.
The tests are easy for this
1 Meter or have someone meter between outdrives there should be no resistance whatso ever (meter the leads to each other first) the copper earth cable between outdrives should be 10mm csa tinned copper wire nothing else will do.
It could have all been caused by a loose anode on one outdrive so after insatlling new outdirves measure the resistance between the anode and the prop. It should be the same reading that you get from connecting the meter leads together.
IF you find that there is zero resistance between both outdrives, or between outdrive to prop on each then it must be stray voltage within the water the vessel is mored, this is easy to check (with the correct meter)
For all these tests you need a top quality meter capable of measuring low resistance and low voltages down to mv's.
How to test your berth for stay currents:
You will need
1 2 legnths of 3mm csa copper wire
2 a volt/multimeter (good quality) capable of reading mv's
3 2 old heavily pitted anodes of any shape odd or even

THis is all you need do:
1 Bare off 30cm of one end of each legnth of wire and wrap it tightly around the midle of each of your anodes and then tape it in place tightly leaving the most parts of the anode exposed.
2 you should now have 2 anodes with a legnth of wire attached.
3 Place both anodes in the water near your berth as close together as you can without them touching.
4 now meter across the other ends of the two wires for millivots you should get a steady low / very low reading.
5 Now move one of the anodes to where your stb beam would be, if you suspect that there is a vessel next to you with a leakage current then leave one anode near the vessel and move the other away. If the voltage starts to increase then repeat the test somewhere else and see if you get the same reading.

If you find that there is a larger voltage relative the the distance between your probes in your berth then you need to have a word with the bloke next door!

The central anode will help protect your underwater metals, angainst electrolysis because they will allways be the least noble metal compared to stainless (especailly stainless) as stainless is way way more noble than zinc. BUT if you have any zinc left at all AND you are still loosing more noble metals underwater then you have a galvanic problem not an electrolysis problem.

Basically, make sure that both legs have no resistance between them if there is then buy some thick wire 10mm and connect them together, then meter them again to be sure. if there is no resistance at all between them and you sitll suffer the same problem (diferencial errosion) then it is most likely that there are stray electrical currents in the water from nearby vessels.
2 the good out drive

 

[Ships_Cat]

Sounds like alot of fun but mostly a load of rubbish /forums/images/graemlins/crazy.gif.

I'm tired of explaining what an electrical circuit is and the myth of electrical currents floating around in the sea electrolyte with no other conductor (has been said there is no shore cable on this boat), "nearby" steel piles, etc - maybe someone else can have a go?

John

 

[duncan]

poole is notoriously bad for anodes on outdrives

the good news is the clear water int he summer to pop over and look at the state of the anodes!

I don't have shorepower, there is non on the pontoons in the area at all. I do have large stainless props and get through 3 sets of anodes a year...............

finally I would have the props checked - tapping them will give you an indication of the damage - no metalic dink and I would certainly remove them and get propellor revolutions in Cobbs to look at them in more detail. They could be severely weakened.

 

[vyv_cox]

It's these "stray electrical currents" that I blame. My lights frequently flash on and off, the engine starts on its own in the middle of the night and my shower tray pump has a mind of its own. Drape the whole boat with electric cable I say, and you can't have too many anodes. I'm putting some up my mast in case the little b**ggers get up there.

 

[vyv_cox]

But more seriously..

you cannot assume that your props have the same manufacturer, composition or metallurgy. In these days of outsourcing they may not have been made on the same continent. So the fact that one corrodes and the other does not is not likely to be a consequence of the installation. I suggest that you fit anodes as near as possible to the props, shaft anodes being the preferred option if your setup allows them. Make sure that the prop is electrically connected to the shaft, and make the anode to shaft connection as good as possible.

Filling the pits in the corroded prop with resin-based filler and then painting will help to keep further corrosion at bay and will improve water flow but, of course, will do nothing to restore any strength lost.